1. Field of the Invention
The present invention relates in general to digital communication systems and in particular to mobile radio systems. Still more particularly, the invention relates to a method of reception and a receiver in a digital telecommunications system.
2. Description of the Related Art
Digital telecommunication systems typically employ one or more modulation schemes to communicate information such as voice, data, and/or control information. These modulation schemes may include GMSK (Gaussian Minimum Shift Keying), M-ary QAM (Quadrature Amplitude Modulation) or M-ary PSK (Phase Shift Keying), where M=2n, with n being the number of bits encoded within a symbol period for a specified modulation scheme.
The EDGE (Enhanced Data rates for GSM Evolution) and the associated packet service EGPRS (Enhanced General Packet Radio Service) have been defined as a transitional standard between the GSM/GPRS (Global System for Mobile Communications/General Packet Radio Service) and UMTS (Universal Mobile Telecommunications System) mobile radio standards. Both GMSK modulation and 8-PSK modulation are used in the EDGE standard, and the modulation type can be changed from burst to burst. GMSK is a non-linear, Gaussian-pulse-shaped frequency modulation, and 8-PSK is a linear, 8-level phase modulation. However, the specific GMSK modulation used in GSM can be approximated with a linear modulation (i.e., 2-level phase modulation with a π/2 rotation). Similarly, the 8-PSK modulation in EDGE is an 8-level phase modulation with 3π/8 rotation. The symbol pulse of the approximated GSMK and the symbol pulse of the 8PSK are identical.
In digital telecommunication systems employing multiple modulation schemes, if the modulation types to be used for information transmission between a transmitter and a receiver is not in a predetermined manner, the receiver must be either informed of the modulation type in advance via some measure or the receiver should be equipped with a capability to detect the modulation type from the received burst signal.
If the communication system is designed to inform the receiver of the modulation type used to send information for each and every burst before the transmission, the communication system will require extra bandwidth to convey the modulation type information to the receiver. In addition, this also introduces system latency, which is not permissible for real-time applications, such as in voice communication. Thus, a system for informing the receiver of the modulation type prior to message transmission is highly undesirable in practical applications.
Another technique is for the receiver to detect the modulation type of a particular burst of information in a digital communication system. Because the receiver relies on the transmission itself to detect the modulation type, this procedure is sometimes referred to as blind modulation detection. One blind modulation detection technique makes use of the training sequence included in every data burst. Common transmission standards define data bursts to include a fixed predetermined training sequence comprising a sequence of symbols, which are known to the receiver. The training sequence is intended to be used by the receiver to estimate the arrival time of the burst and the distortion and noise characteristics of the transmission channel. For example, upon receiving a training sequence, the receiver correlates the received training sequence signal to the known signal of the same training sequence. The receiver then utilizes this correlation to characterize the arrival time and channel (i.e., estimate the channel effects). In GSM wireless communication systems, for example, information is transmitted in transmission bursts, wherein each transmission burst may consist of two sections of data bits with a 26-bit midamble (training sequence) located in between. According to the GSM technical standards, one of eight possible training sequence codes can be used as the midamble.
The blind modulation detection technique using training sequence in a digital communication system derives a metric from the received training sequence signals for each possible modulation type. Each metric represents a likelihood that the corresponding modulation type is used by the transmitter to create the received signal. The metrics are analyzed based on certain decision logic to determine the modulation type of the received burst signal. As is known, GMSK and 8-PSK modulation in EDGE are distinguished by using different symbol rotations. All GMSK modulation rotates each transmission symbol by π/2 relative to its prior symbol in addition to the information phase, while the 8-PSK modulation rotates each transmission symbol in additional to the information phase by 3π/8 relative to its prior symbol.
One method of deriving a metric passes the received signal into data path that rotates the signal back through a corresponding phase rotation for each symbol, called de-rotation. For example, in EDGE communication system where there are two possible modulation types, GMSK and 8-PSK modulation, the received training sequence is rotated in a first data path by −nπ/2, where n is the symbol index in the sequence, and in the second data path by −n3π/8, respectively. After this, the de-rotated received training sequence is compared with a known training sequence stored in the receiver to generated two metrics, each with a different phase rotation assumption. The metric can be calculated by magnitude squared correlation of the de-rotated received training sequences with the known training sequence. Then the transmission modulation type is determined by the maximum of the two metrics.
In GSM communication systems, one way to increase system capacity is to increase the frequency reuse factor, whereby the communications system allocates the same frequency to multiple sites in closer proximity. However, when proximate cell sites transmit within the same frequency band, co-channel interference can occur, and when devices transmit in adjacent bands, adjacent-channel interference can occur if sufficient inter-band spacing is not provided. As a result, increased frequency reuse increases the co-channel interference and adjacent channel interference. Therefore, receivers operating in such an environment are required to have a better interference rejection performance.
Single Antenna Interference Cancellation (SAIC) is a general term used for advanced communications systems and receiver algorithms designed for the purpose of improving system capacity through increasing frequency reuse by enhancing single-antenna receiver performance in the presence of co-channel interference. SAIC is a promising technology currently being standardized in the industry that appears to be an attractive solution to the problems of frequency reuse, even though SAIC increases the complexity of the receiver. In the most advantageous interference conditions, SAIC can improve the signal-to-noise ratio over 10 dB. Current SAIC receiver algorithms are generally optimized for GMSK modulated signals, since gains of SAIC tend to be smaller for 8-PSK modulated signals. In an SAIC operational environment, GMSK traffic on neighboring cells can reuse common frequencies, thereby significantly increasing network bandwidth, while still tolerating the significantly higher co-channel and multi-channel interference than has been previously seen in conventional GMSK/EDGE environments.
While use of SAIC in wireless telecommunication systems overcomes the receiver performance issues introduced by frequency reuse, the high interference in a SAIC environment significantly impacts the reliability of traditional modulation detection techniques such as the prior art systems described above. Ideally, the receiver should be able to determine the modulation scheme associated with a particular burst of information regardless of the operating environment. However, the increased co-channel and adjacent channel interference created by SAIC operating environments does not permit conventional modulation detection techniques to reliably detect the modulation type of a transmission burst, thereby degrading radio telephone performance and quality. Further compounding the problem is the fact that front-end modulation detection must be capable of attaining the same or greater signal gain as attained by the SAIC receiver in order to fully achieve the large gain advantages from SAIC receiver. Unfortunately, prior art modulation detection methodologies fail to achieve the necessary gain because of the higher co-channel interference in SAIC environments. Therefore, what is needed is a new modulation detection technique that reliably performs and achieves high gain in the SAIC operational environment to achieve full performance of the receiver implementing SAIC.